The present invention relates to a reflective liquid crystal display and, more particularly, to a reflective liquid crystal display capable of providing a bright display, and a holographic reflector used in this apparatus.
The entire contents of Japanese Patent Applications No. 8-142895 filed on Jun. 5, 1996, No. 8-184446 filed on Jul. 15, 1996, and No. 8-304626 filed on Nov. 15, 1996 are incorporated herein by reference.
Liquid crystal displays include a reflective liquid crystal display in which a reflective layer is arranged on the back surface of a liquid crystal panel to avoid the use of any backlight. In this reflective liquid crystal display, light (external light such as indoor illumination or sunlight) from the observer side is reflected by the reflective layer through the liquid crystal panel, and reflected light corresponding to a pattern displayed on the liquid crystal panel travels toward the observer.
Recently, in the reflective liquid crystal display, the use of a reflection hologram as the reflective layer in place of the existing metal reflective layer has been examined. In the reflective liquid crystal display using such a reflection hologram, the viewing range and the reflection direction of reflected light can be specified, and a brighter display in a specific direction can be realized compared to the display apparatus using the metal reflective layer.
When, however, the reflection hologram in use is a surface relief type hologram, the diffraction efficiency is difficult to increase. Moreover, the color sensed by the observer changes in accordance with the observation direction due to color dispersion of the applied hologram.
When the reflection hologram in use is a volume type reflection hologram, the width of the wavelength to be reflected and diffracted due to wavelength selectivity is small. For this reason, reflected light colored (in a specific color except for white and silver) is undesirably sensed. A bright display throughout all the visible wavelength region is difficult to realize.
To attain a color display in the liquid crystal display, a well-known color filter of, e.g., pigment dispersion type is employed together with the liquid crystal panel. In the reflective liquid crystal display, however, the display brightness decreases due to absorption of light by the color filter, and the cost increases.
It is the first object of the present invention to provide a holographic reflector capable of realizing a bright display in a reflective liquid crystal display using a hologram as a reflective layer, and the reflective liquid crystal display using it.
It is the second object of the present invention to provide a holographic reflector capable of controlling light having a large reflection/diffraction wavelength band width and a high diffraction efficiency to attain a bright display in a predetermined direction in a reflective liquid crystal display using a hologram as a reflective layer, and the reflective liquid crystal display using it.
According to an aspect of the present invention, there is provided a holographic reflector comprising: a volume type transmission hologram having angle selectivity; and a reflective layer arranged on one surface of said volume type transmission hologram.
In this manner, since the reflective layer is arranged on the back surface of the volume type transmission hologram, the holographic reflector can have a high diffraction efficiency (reflectance of external light) and a large diffraction wavelength band width. In addition, a bright display can be attained within a predetermined range corresponding to the focusing function of a hologram lens.
Since the hologram is a volume type transmission hologram having angle selectivity, incident light (or light reflected by the reflective layer) is diffracted by the hologram to emerge as reflected light in a direction different from the regular reflection direction of the incident light. The observer does not sense an image of the light source for the incident light formed upon reflection on the hologram surface, and the viewing range and the reflection direction can be specified.
The angle selectivity is an optical property of the hologram in which only light incident at a specific angle (direction) leaves at a specific angle (direction) as diffracted light. The specific angle (direction) is determined in accordance with the conditions in recording the hologram.
A plurality of different types of hologram lenses can be (1) multiple-recorded on a single holographic material, (2) integrated by stacking, or (3) recorded in areas divided into patterns on a single holographic material.
With this arrangement, the direction of the incident light (external light) which contributes to reflection is hardly limited. Even if the observer moves his/her viewpoint, he/she can satisfactorily sense the reflected light. In this manner, the range of observation conditions which allow the observer to sense bright reflected light can be widened.
In this case, xe2x80x9ca plurality of different types of hologram lensesxe2x80x9d transmit (1) a single reconstruction illumination beam to emit a plurality of types of diffracted beams with different focal point positions, or (2) different reconstruction illumination beams to send diffracted beams to the same focal point position.
As described above, a holographic reflector capable of realizing a bright display in a reflective liquid crystal display using not a metal reflective layer but a hologram as a reflective layer, and the reflective liquid crystal display using the reflector can be realized.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.